As I recall, in some of those cases, the carboy was said to have broken without obvious cause - not dropped or bumped. I wonder if the reason might be, at least in part, weakened glass due to cleaning with strong solutions of caustic cleaners, or adding powdered cleaner to an empty carboy. This is described in a report from the Better Bottle company:
"Washing and Sanitizing Home Winemaking and Brewing Equipment", available on their website:
(click Technical -> Wash/Sanitize - click on link in second paragraph).
"AN IMPORTANT WARNING
Glass, soft glass in particular, is slightly soluble in water and more soluble if the water contains caustics, which are common ingredients in washing agents. As glass dissolves from the surface of glass objects, micro cracks form and the strength of the object becomes increasingly compromised. With continued use, glass carboys become more and more fragile. Thus, a mode of handling that you may have executed many times with a given carboy can result in an unexpected breakage. Never carry glass carboys by their necks and be aware that even lifting a full glass carboy off a counter by placing your hands around its base can cause it to crack wide open. Carboy carrying harnesses are not an ideal solution, as they do not provide protection from flying glass shards. Carrying bags and crates offer greater protection. Washing glass carboys is especially dangerous, because they will be, of necessity, uncovered and likely to be slippery – always wear protective gloves and clothing."
Of course this was placed on their website to encourage people to buy their product, but that doesn't mean it isn't true.